The present invention relates to a method of making a solar energy collector element and more particularly, to such element which is superior in selective absorption characteristics for the solar energy and inexpensive in manufacturing.
There have been developed and practically used solar systems such as electric generator systems, solar houses and others which utilize the solar energy as a source of substantial non-pollution energy instead of energies from coal and petroleum. These solar systems utilize solar energy collector elements each including a solar energy collecting film or surface (called selective absorption film or surface) formed at the side of receiving the sunlight, the solar energy collecting film or surface having characteristics that it is high in absorption efficiency of the solar energy and less in release of the stored heat energy. It has been believed that ideal selective absorption film or surface has such spectral characteristics that the reflection factor is lower for electromagnetic waves in a zone including wavelengths in the range of 0.3 to 2.5 .mu.m which is mainly of visible rays and near infrared radiations and includes most of the solar energy, and higher for electromagnetic waves in another zone including wavelengths over 2.5 .mu.m which is mainly of infrared radiation and includes most of radiation energy on heat reserve. That is, such spectral characteristics provide a reduced rate of radiation.
There have been known the following methods of making solar energy collector elements which have such a selective absorption film or surface;
(i) A method of treating a substrate of aluminium into black color by the steps of anodizing the aluminium substrate and subjecting the aniodized substrate to secondary electrolysis to deposit copper, nickel and others thereon (Japanese provisional patent publication No. 20195/1981), a method of forming a film on the surface of a copper substrate with the film being of copper oxide or black chromium, black nickel or the like which is formed by electroplating (Solar Energy, Vol. 19, 1977; J. Vac. Sci. Technol., Vol. 13, No. 2 Mar./Apr., 1976), and a method of vapor-depositing a layer of aluminium on a glass plate and then coating on the aluminium layer with a film of chromium oxide (Japanese provisional patent publication No. 87738/1977),
(ii) a method of applying a coating material containing a pigment of semiconductor particles of silicon or the like to a metal surface of Al, Fe, Ag, Ni, Cr or others (Japanese provisional patent publication No. 146353/1980), and
(iii) a method of forming a coherent multilayered film on a metal surface by the use of vacuum deposition or chemical deposition (Thin Solid Films, Vol. 39, 1976, P. 87).
Solar energy collecting films or surfaces made by the above methods (i) and (ii) are of transparent semiconductor films or surfaces which quite absorbs visible rays and near infrared rays. Accordingly, such films or surfaces have satisfactory selective absorption characteristics for the sunlight. However, they are inferior in absorption efficiency for infrared rays because of the transparent semiconductor films or surfaces. The above methods (i) are disadvantageous in that they are higher in manufacturing cost because of the expensive materials used such as aluminium and copper and that they are troublesome in process because of the treatment such as deposition and plating, resulting in inferior mass-productivity. They also have problems in pollution due to the waste liquid exhausted on chromium-plating and others. The above method (ii) has a disadvantage in that the solar energy collecting surface made according to this method has higher surface reflection because silicon particles used as a pigment have higher refractive index for the sunlight, resulting in reduced absorption efficiency. Further, the above method (iii) provides such a product that is generally lower in selective absorption characteristics, but must be produced according to a complicated process while being improved in above-mentioned surface reflection.
In order to overcome the problems in the aforementioned methods, there has been developed methods wherein metal materials of relatively inexpensive irons are used as substrates. These methods include a method of annealing a sheet of steel to deposit the carbon on the steel in the form of graphite (Japanese provisional patent publication No. 156434/1977); a method of forming a film of Fe.sub.3 O.sub.4 on the surface of carbon steel or the like by dipping it in an alkaline solution which is produced, for example, by dissolving sodium chloride, sodium phosphate, sodium nitrite or the like into distilled water or demineralized water (Japanese provisional patent publication No. 75132/1978); a method of causing a chemical treating liquid containing copper ions to act on the surface of a substrate made of an iron metal material and treating the surface into black color as well as depositing fine metal copper particles (Japanese provisional patent publication No. 84240/1978) and so on.
However, the characteristics of selective absorption films or surfaces obtained according to these methods were not necessarily satisfactory.
Namely, the above method of depositing the graphite by annealing must use steel sheets of high carbon content and the obtained products have insufficient selective absorption characteristics. Such steel sheets are inferior in workability and welding property. Accordingly, it is difficult to obtain the desired solar energy collecting plate which is satisfactory in thickness. The above method of dipping the carbon steel in the alkaline solution has problems in safety of operation and control of bath because the alkaline solution used is concentrated. The above method of depositing copper is disadvantageous in that management of bath is troublesome since the chemical treating liquid containing copper ions is used therein and that the waste liquid provides a source of pollution and so on.